1. Field of the Invention
This invention relates generally to power conversion apparatus utilizing resonant dc converter topology for dc power supplies, and more particularly, to dc power supplies utilizing resonant dc converter topologies that provide a wide load range at full power for application to plasma processing.
2. Brief Description of the Prior Art
DC power supplies have found wide spread use in plasma processing applications such as plasma sputter deposition of thin films. It is advantageous if the power supply is able to deliver maximum power to the plasma over as wide a range of plasma load impedances as possible. U.S. Pat. No. 5,535,906 issued to Geoffrey N. Drummond assigned to the assignee of the instant application that teaches a multiphase L-C-C resonant power supply for plasma loads. A simplified schematic diagram of a converter following the teachings of U.S. Pat. No. 5,535,906 is shown in FIG. 6. This type of power supply can typically deliver full-power over about a 4:1 range of load impedances, but the range of load impedances that may be presented by plasma loads at given power level may be much greater. Consequently, several models of a power supply may be required to accommodate the range of plasma load impedances. There has been a long-felt need for a power supply which can provide full power over a broad range of load impedances to consolidate the number of power supply models that are required to operate various plasma loads.
The full-power output impedance range of a power supply is primarily determined by losses in the components. The losses in a resonant power supply for a given output power level and a given dc supply voltage are inversely related to the power factor seen by the inverter switches. In FIG. 6, switches 96, 98, 100, 103, 104 and 106 represent electronic switches such as field-effect transistors (FETs). Each switch may also include diodes connected to prevent current from flowing in the body diodes of the FETs. The power factor seen by the inverter switches may be defined as the cosine of the phase angle between the fundamental component of the square-wave voltage produced by a pair of switches (e.g. 96 and 98), and the fundamental component of the current flowing out of the node where the switches are connected to each other (e.g. the current through capacitor 108.)
There is an optimal load resistance for which the power factor is closest to unity. The power factor is reduced as the load resistance varies from the optimal value. This effect is plotted in FIG. 9 for an implementation of the prior-art power supply of FIG. 6, and also for embodiments of the present invention. Modifying a power supply circuit so that the range of load impedances for which the inverter power factor is relatively high is broadened increases the full-power impedance range of the power supply.
U.S. Pat. No. 5,874,788 issued to Thomas McCartney teaches the use of a rectifier circuit at input of ac-dc power supplies that can be switched between normal and voltage-doubling modes in order to broaden the range of ac mains voltages for which the power supply can properly operate. A pair of series-connected capacitors 76 and 78 are connected between dc output terminals 80 and 82 of a bridge rectifier consisting of diodes 66, 68, 70 and 72, as illustrated in FIG. 3. Switch 74 allows the circuit to be adapted to differing ac input voltages applied between ac input terminals 62 and 64. When switch 74 is open, the dc voltage between output terminals 80 and 82 is approximately equal to the amplitude (peak value) of the ac mains voltage. Closing switch 74 approximately doubles the dc voltage between output terminals 80 and 82. Switch 74 is closed for a 120 volt ac input voltage and open for 240 volts ac. Consequently, the voltage between the dc output terminals remains relatively constant for both 120 volt and 240 volt ac mains voltages. The values of capacitors 76 and 78 are selected to be large enough so that the voltage-doubling function is achieved over the intended range of load resistances.
It is an object of this invention to provide a resonant dc power supply that has a wide output impedance range. It is also an object of this invention to provide a wide range dc power supply that utilizes passive networks with a minimum number of parts and has low cost. In furtherance of these objects, there is provided a novel dc power supply design that utilizes a capacitive network that modifies the output range of conventional converter topologies wherein load impedance, converter frequency and output interactions provide high inverter power factors and low circuit losses over load impedance ranges as high as 64:1. Selectively doubling the output voltage electronically with capacitors for high impedance loads or halving it electronically for low impedance loads with an inductive choke can extend the full power load impedance range of a power supply as taught herein.
Both single-phase and multiphase circuits are taught that place capacitors in parallel with rectifying diodes wherein the capacitors have little effect for low values of load resistance.